Portable devices are becoming more common in every aspect of our lives. For example, many consumers listen to their music from portable audio players. These portable devices become easier and more enjoyable to use when they are provided in small packages, offering the same capabilities while still having long lasting battery charge. To support these longer lasting devices, the operating voltage of many of the components inside is often reduced to reduce the power consumption. However, this reduction in operating voltage may affect the sound output of the portable device as the volume level of an audio signal is proportional to the output voltage. Boost converters may therefore be used to increase the operating voltage for select components within a portable device that would benefit from higher operating voltages, for example, audio amplifiers which may require high voltages to provide high volumes.
FIG. 1 illustrates a conventional audio amplifier comprising a boost converter. The audio amplifier comprises a battery supply 102 which is configured to provide a voltage Vp to a boost converter 104. The boost converter 104 increases the voltage Vp to a boost voltage VBST. In this example, a speaker amplifier 108 is configured to receive the boost voltage VBST and to output an analog audio signal, SIG, from a digital to analog converter (DAC). The speaker amplifier increases the low power signal, SIG, with the power received from the boost converter to generate a signal to drive a speaker 110. The boost voltage VBST is higher than the supply voltage Vp which allows the speaker amplifier 108 to provide louder audio signals through the speaker 110 than would be possible using only the supply voltage Vp.
However, generating the boost voltage VBST may have some drawbacks such as increased power consumption of the audio amplifier 100. In other words, as the speaker amplifier is receiving the boost voltage VBST then, even in circumstances where the volume levels are low, the speaker amplifier is wasting power compared to an implementation where it only receives Vp. This wasted power may shorten the operating time of the device comprising the audio amplifier between charges, and may be detrimental to the user experience of the device.
In portable devices, operational battery life for a given charge cycle may be considered critical. Consumers may request that end manufacturers produce longer times between battery charging cycles. This request then may encourage the end manufacturers to require original equipment managers (OEMs) and hardware developers to reduce operational power consumption in their sub-systems wherever possible, in order to extend battery life between charging cycles. Boosted amplifiers may often be one of the heavy power consumers in a portable device containing audio amplification. Managing the operational transitions between discontinuous conduction mode (DCM) and continuous conduction mode (CCM) is one method of managing power losses. CCM operation may be more efficient than DCM when delivering higher output power. However, if the load requirements on the boost are reduced, the average current through the boost converter may also be reduced. However, CCM operation may push and pull current through the inductor even when no power is consumed by the load. This operation may produce losses through the inductor, for example, switching losses, and cross conduction losses which may be unnecessary to maintain the boosted supply voltage (VBST) during low loading conditions. Transitioning over to DCM operation may therefore allow the system to significantly reduce these losses while operating in a low or idle power loading conditions. Conversely, if the load on the VBST supply is larger, operating in DCM may be less efficient than operating in CCM mode, and therefore the boosted system may operate in CCM mode in order to increase battery operational life for a given boosted amplifier load.